1. Field of the Invention
The present invention relates to a lathe.
2. Description of the Related Art
A lathe that machines a work in combined manners is called combined-machining lathe. A combined-machining lathe is disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. H11-138374, and Unexamined Japanese Patent Application KOKAI Publication No. 2003-117701.
The lathe disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-138374 is a combined-machining lathe whose main spindle is fixed. This combined-machining lathe has a main spindle for gripping a work, a tailstock, a reciprocating carriage, and a tool spindle. The tailstock is arranged at a position opposite to the main spindle. The reciprocating carriage is a carriage that reciprocates in parallel with the centerline of the main spindle or of the tailstock. The tool spindle is fixed on the reciprocating carriage. The tool spindle grips a tool. The tool spindle can change the direction of the tool. A work that is gripped by the main spindle or the tailstock is machined by the tool.
When a lengthy work is machined by a combined-machining lathe having such a configuration as described above, the work extends from the main spindle with its one end gripped by the main spindle. Due to this, the work is pushed by the tool and bent. Precise machining cannot be applied if the work remains bent. Therefore, for machining a lengthy work, it is necessary to attach to the tailstock, a center that would abut on the end surface of the work to support the work in cooperation with the main spindle.
However, the following problems arise when a center is attached to the tailstock,
(1) Since the center is applied to the end surface of the work, the end surface (front surface) of the work cannot be machined.
(2) Since the center is attached to the tailstock, a chuck for gripping the work cannot be attached to the tailstock. Therefore, it is impossible to machine the back end surface (back surface) of the work with the work gripped by a chuck at the tailstock.
(3) Since there is a fear of the tool and the tailstock interfering with each other (colliding on each other), a portion of the work that is held close to the tailstock cannot be machined.
As described, with a lathe whose main spindle is fixed, it has been difficult to machine a lengthy work in combined manners.
Further, with the combined-machining lathe disclosed in the Unexamined Japanese Patent Application KOKAI Publication No. H11-138374, length of work that can be machined is restricted depending on the distance between the main spindle and the tailstock. Therefore, a work having a length equal to or larger than the distance between the main spindle and the tailstock cannot be machined.
On the other hand, a method of machining a lengthy work without using a tailstock is disclosed in the Unexamined Japanese Patent Application KOKAI Publication No. 2003 -117701. Unexamined Japanese Patent Application KOKAI Publication No. 2003-117701 discloses a technique for providing a guide bush for preventing bending of the work adjacently to the tool, and machining the lengthy work by moving the main spindle.
The lathe disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2003-117701 has a first headstock for rotatably gripping a work, a second headstock and front tool rest which are arranged on a back headstock, a guide bush, a tool rest to which a bite for cutting a work is attached, a third headstock, a tool attached to the third hardstock for machining the back surface of a work, and a rotatable tool carriage which is movable perpendicularly to the axial direction of a work.
The back headstock has a function for moving in the direction of axis of a work which is gripped by the first headstock, and a function for moving perpendicularly to this axial direction. The guide bush supports the work which is rotating. The second headstock grips the end surface (front surface) of the work. A drill for machining the front surface of the work is attached to the tool rest. The third headstock is arranged at a side of the guide bush.
According to the lathe disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2003-117701, the front end (front surface) of a work is machined by a tool attached to the front tool rest while the work is gripped by the first headstock and further supported by the guide bush. After this, the back headstock is moved, so that the front end surface of the work is gripped by the second headstock and then the work gripped by the second headstock is gripped by the third headstock. Sequentially, the front end of the work gripped by the third headstock is machined by the tool attached to the tool rest.
In a case where the back end (back surface) of the work is machined by this lathe, the back headstock is moved, so that the back end (back surface) of the work gripped by the second headstock is machined by a drill attached to the third headstock.
The lathe disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2003-117701 can machine the front end and back end of a lengthy work, but has the following problems.
(1) In order to form an angled hole in the work, it is necessary to attach an angled hole forming attachment to the tool rest.
(2) It is necessary to separately prepare a tool for machining the front end surface of the work and a tool for machining the back end surface thereof.
(3) When positioning a tool on the tool rest, it is necessary to consider a fear that the tool might interfere with (collide on) an adjacent tool.
(4) It is necessary to secure in the lathe, a place where the tool for machining the back end surface of the work is attached, in addition to a place where the tool for machining the front end surface of the work is attached. This makes it difficult to downsize the lathe and also deteriorates the working efficiency.
(5) The number of tools that can be attached to the tool rest and the third headstock is limited. This limits the kinds of works that can be machined in a complex manner without changing tools.
(6) It is necessary to stop the lathe when changing tools, making the working efficiency worse.